1. Field of the Invention
This invention relates to a multiple speed ratio automatic transmission. More particularly, the invention pertains to such a transmission having multiple countershafts that are selectively coupled to the crankshaft of an engine.
2. Description of the Prior Art
Conventional automatic transmissions whose gear arrangement is of the epicyclic type employ torque converters between the engine crankshaft and the input shaft of the transmission. Torque converters and hydraulic couplings operate with an impeller driven by the engine that produces a toroidal circulation of hydraulic fluid between the impeller and a turbine. The change of momentum of the circulating fluid drives the turbine.
A hydrodynamic torque converter is a device for converting torque, or turning moment, by making use of the kinetic energy of fluid in motion. The impeller imparts a whirling motion to the fluid, a runner is kept rotating by the whirling fluid and the reaction member or turbine changes the direction of the whirling motion between the runner outlet and impeller inlet so that the kinetic energy of the fluid drives the impeller. When starting from rest the efficiency of the torque converter is zero and can range as high as 85-90 percent at the higher speed ratios. Over a considerable portion of its operating range, particularly near its lower speed ratios, the converter operates at less than 40 percent efficiency. Lock-up clutches used in torque converters are capable of improving only slightly the efficiency from the high values to 100 percent in the high-speed ratio range.
The hydraulic control circuitry that produces the multiple speed ratios of the epicyclic automatic transmission is an additional source of power loss since the fluid must be pressurized in order to actuate the clutches and servos that produce the several speed ratios. Furthermore, the hydraulic circuit must continue to be pressurized to positively hold these components while the speed ratios are required.
Heavy vehicles such as trucks that use manual transmissions often require a downshift into a lower speed ratio while the vehicle is attempting to climb a hill. Manual transmissions require that the engine torque be removed from the transmission during the period when the higher speed ratio is disengaged and before the lower speed ratio is engaged. During this time the drivewheels of the vehicle receive no power; consequently, the vehicle loses an appreciable portion of its momentum on a hill. Once momentum is lost the driver often must select the lowest speed ratio of the transmission and creep up the hill while in the lowest speed ratio unless sufficient momentum is regained to sustain the vehicle while an upshift is made. It is preferred that a transmission, whether automatic or manually operated, avoid this period during which the wheels receive no power so that fast engagement of a lower speed ratio can be made. Preferably this advantage can be realized in a transmission that operates more efficiently than a conventional automatic transmission.
It is often required that a stopped vehicle, whose drivewheels have lost frictional contact with the road, be slowly and gently shifted between the first forward ratio and reverse drive in order to clear an obstruction or to regain driving contact with the road surface. Present day manual and automatic transmissions, in order to produce this shifting action, require that the synchronizer clutches, or in the case of an automatic transmission, the clutches and servos that operate to produce the first and reverse drive ratios, be engaged each time the respective gear is selected. It is preferred that this rocking action results merely by preselecting and engaging the transmission components that connect the gear ratio elements to the output shaft and then, by operation of clutches, connect the output shaft through these gears to the crankshaft of the engine.